The invention relates to aspects regarding a method of registering lightning strikes in the blade of a wind turbine, a system of exercising the method, exercise on a wind energy plant and a suitable resistor element in the same context.
It is commonly known to equip high structures, such a high-rise buildings, chimneys, antennae and wind energy plants with lightning grounding devices. By means of such lightning grounding devices it is ensured that lightning current is grounded in a responsible manner and without damage to the structure.
In case of eg the blades of a wind turbine, a strike of lightning can be captured by means of receptors, as described more specifically in eg WO 0079128 and WO 0248546, incorporated herein for reference. Essentially, receptors are metallic elements arranged strategically on the blade and being in connection with one or more lightning grounding cables, through which the lightning currents are grounded.
In order to enable estimation of the force of a lightning strike in the blade of a wind turbine, a so-called lightning registration card of a size similar to that of a credit card can be mounted in the blade, as disclosed in EP 0 663 598, wherein a magnetic field generated by a lightning leaves traces on the magnetic strip of the lightning registration card. In this manner it is possible to read the maximal lightning current to which the blade was exposed.
In order to enable this reading, however, it is necessary to stop the wind energy plant and dismount the lightning registration cards in question that may be mounted eg within each blade of the wind turbine. The lightning registration cards can subsequently be read by means of special equipment, whereby the maximal value of the lightning current can be determined. However, it is not possible to determine neither the duration of the lightning strike nor when it happened.
Precisely the duration and point in time of the lightning strike are desirable items of information—on the one hand to enable assessment of the risk of damage, if any, to the blade and, on the other, in connection with insurance issues concerning blades of wind turbines that have been damaged by lightning strikes.
If a lightning current of a certain magnitude—eg 50 kA—is discharged during a few microseconds, ie the duration of the lightning current is quite brief, it is likely that no damage is done, as there has not been time enough for any heating worth mentioning of the blade and the air contained in the blade to have occurred. This is assumed to be linked with the fact that, obviously, when the duration is short, the energy amount is limited. Conversely, in case of a less powerful lightning current of eg 30 kA discharged during a full second, a more powerful heating takes place with ensuing possible damage to the blade. The reason for this is assumed to be the discharge time which results in the amount of energy being many times greater than the more powerful strikes of lightning of shorter duration mentioned earlier.
It is known to measure lightning current and discharge time by means of an oscilloscope or digital recording equipment; however, this equipment is expensive laboratory equipment which is moreover not suitable for mounting in eg the blade of a wind turbine.